Lesson · Problem 6
Choose the Right Trace
Width follows the stated limit
You should be able to
- Use the stated minimum width on every segment of a net.
- Refuse a connected trace that is narrower than the net’s limit.
Routing · PCB Fundamentals
What it is
Trace width is the copper’s drawn width on that layer. Every segment of a net has to meet the limit. A via is not a substitute for a wide trace on the layer the rule names. The brief’s currents and the 35 µm copper note are context. They are not a second, hidden score.
Why this matters
A narrow trace has more resistance and runs hotter for the same current. This challenge grades the width you were given, not a calculator you invent.
What you are building
Three nets, each with its own minimum width, on a board that is otherwise a routing exercise.
Prerequisites
This lesson uses trace, net, clearance, board outline, via. It introduces trace width, copper cross-section, segment width.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Trace
- What it does
- A strip of copper of a stated width on a stated layer.
- Why it is here
- Width is the knob this challenge grades. Current in the brief is context, not a calculator you must run.
- Symbol
- There is no new schematic part. The copper is the object.
- Constraints
- SIGNAL, AUX, and POWER each have a minimum width on every segment.
- Beginner mistake
- A wide pour that still hides a narrow neck. Every segment counts.
- What an engineer checks
- The width the brief states for that net. This is not a temperature-rise table.
Interview lens
If asked about current, say width and length both change resistance, then use the width this brief published. Do not invent a temperature.
What PCBGrade measures
Every segment of SIGNAL, AUX, and POWER against its minimum. A neck fails. Current in the text is context.
Where this shows up
A power net and a signal net do not share a width just because the same tool drew them.
Terms
- Trace width
- The drawn width of the copper segment, in millimetres.
- Copper weight
- How thick the foil is. Here it is context, not a separate graded calculation.
What is happening electrically
A trace is a resistor made of copper. Its resistance grows with length and shrinks when the cross-section grows. Cross-section is width times thickness. The same current in a narrower trace drops more voltage and heats the copper more.
Why the geometry matters
Width is a property of every segment, not of the net name in the toolbar. A wide segment in the middle does not rescue a narrow neck. A via carries the net to another layer. It does not widen the copper you left behind on this layer.
How an engineer reasons
Read the minimum for that net and set the width before you draw. POWER is asked to be wider than SIGNAL because the brief assigns it more current. Check the whole path, including the last segment into the pad.
Worked example
SIGNAL at least 0.25 mm, AUX at least 0.50 mm, POWER at least 1.00 mm, on every segment. Clearance is 0.20 mm and copper stays at least 0.50 mm from the board edge.
Good and bad
POWER drawn at 1.00 mm from end to end meets the width rule. POWER drawn at the default 0.25 mm can still be fully connected and still fail. Connectivity and width are different measurements.
Common mistake
Sizing every net to the default 0.25 mm because the tool opened there.
Where this rule stops
This challenge grades the widths it published. It does not ask you to run an IPC-2152 temperature calculation, and the 35 µm note is context rather than a second score. Outside this board, width also depends on copper weight, allowed temperature rise, and whether the trace is on an inner layer.
Before the challenge
The challenge publishes a minimum width per net. Apply it to every segment. A connected trace that is too narrow still fails.
Ready for the challenge
You can set a width before you route and check that no segment dropped back to the default.
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